Tortoiseshell, because of its rich hues and distinctive translucent quality, has always offered an attractive medium for craftsmen, and as such, has been a prized ornamental material from very early times. It was found that marquetry using tortoiseshell in Asia Minor dates as early as 350 BC.
In Greece, turtles were the symbol of a space of humanity between the sky (the shell) and the earth (the turtle’s feet). A turtle was considered to own the power for humans to access the gods’ secrets. Tortoiseshell, from sea turtles, was brought to Ancient Rome from India, Egypt, Africa and even from Phoenicia at a precise time in history.
In Japan, the first turtle-shell workshop dates as far back as 16th century Nagasaki. The first trial of marquetry using tortoiseshell, wood and metal was probably made in Portuguese India at the end of the 16th century.
For its rarity and its beautiful colours, tortoiseshell became highly prized in Europe, and an example of that is Henri IV’s cradle, that was made of an entire Hawksbill tortoiseshell, which can still be seen at Pau’s Castle in France.
King Henri IV’s cradle, made from one entire tortoiseshell, 1553.
Later in the 17th century, in France, Pierre Gole, ebeniste du Roi,and later Charles Andre Boulle (1642-1738), the latter being the main artist who achieve great skill in making furniture with marquetry using tortoiseshell, wood and metal; used this material to achieve remarkable furniture.
Tortoiseshell is the carapace of a sea turtle, the main species sought after being the Hawksbill sea turtle, though the Green sea turtle was also used later in the 19th century. The hawksbill tortoises are the smallest of all sea turtles, and can primarily been found in tropical coral reefs.
The shell is thin, flexible and highly coloured with elaborate patterns. The carapace of the hawksbill is unusual amongst the marine turtles as the scutes (the hard, bony plates that constitute the shell) are overlapping. These are often streaked and marbled with amber, yellow or brown, most evident when the shell material is worked and polished.
Their shell is an interesting material because it is a living substance, made of keratin. It is an insoluble protein composed of 55% carbon, 20% oxygen, 16% nitrogen, 6% hydrogen, and 2% sulphur. The shell thickens with age of the turtle, and can get to 8 mm for the Hawksbill turtle. Its density is of 1.29 and its hardness on Mohs scale is 2.5. The reason why it is the Hawksbill turtle that was mainly used in France in the 17th and 18th centuries is because of its very thick shell and rich colours.
Moreover, another very interesting property for furniture makers is the discovery that the shell has the characteristic of being able to auto- graft itself. The discovery in the 19th century of the possibility of auto-grafting enabled a higher volume of work, and considerably extended the applications. Turtle-shell could be welded, turned, sculpted, shaped and this enabled craftsmen to create work of lace-like dexterity.
The green turtle tortoiseshell (chelonia mydas), is much thinner and little coloured for the adult turtle, offers less interests and has much more difficulty to auto graft itself. Though the preferred choice was to work with hawksbill tortoises, the 19th century under Napoleon III’s reign, saw a massive use of green turtle tortoiseshell to make veneer, mainly because of its low price and easy supply compared to hawksbill.
The France of Louis XIV saw the golden age of turtle-shell in particular thanks to the cabinetmaker Charles-André Boulle whose technique resulted in some of the finest masterpieces of the art of furnishing that included consoles, desks and tables, hanging wall clocks, coffers and cabinets.
From 1640 onwards, tortoiseshell was employed as a veneer, with a backing of red pigment or gold foil in order to simulate amber, and the hard translucent material that is the tortoiseshell would usually be yellow and brown. Also, the tortoiseshell could be, when heated, moulded as the desired shape, and would then harden as the furniture maker would have intended it. The underneath of the tortoiseshell is always blond compared to its back, and the perimeter, known as fry, was never used because of its fragility, being too thin for efficient working.
Following Boulle work, tortoiseshell became popular in England as well, where it was worked to produce long case clocks, and later in the Victorian era (1820), to replicate Boulle’s furniture.
Pierre Gole started to develop a new technique to work with tortoiseshell for the King, and he is known to have produced a table for the Chateau de Vincennes for Mazarin in 1663. However, Boulle made his particular technique a known skill. His superb technique for associating wood work, gilt and tortoiseshell contributed to his designation as ebeniste du Roi in 1672. Nevertheless, the juxtaposition of brass and tortoiseshell on wood needed a new technique. Recent research demonstrated how he would work on those materials.
Boulle, and Varin, 1678/1680 Oak veneered with ebony, pewter, tortoiseshell, pewter, brass, ivory, horn, and various woods; with drawers of snake wood; painted and gilded wood figures; bronze mounts.
Boulle’s technique involved putting a thin slice of white linen to preserve the impact of colours and then work the underneath of the materials in order to make them stick better, either by two prong crimping or toothing plane. He also needed to perform a chemical treatment of the brass with acid or mercury. Then, going from the wood surface, he would apply animal glue with black calcinations and white paper together with a coat of animal glue of brown or redish colour. The finishing touch would be to apply a layer of aurea pellis (known as german gold), that enters in contact with the reverse of the tortoiseshell, glued with colourless animal glue. Only then the maker would apply the tortoiseshell.
Furthermore, before applying the tortoiseshell, it needed to be polished, by scraping the surface to smooth out irregularities before sandpapering with a very fine paper so as to avoid scratches, and polishing with a mildly abrasive mixture of powdered rotten stone and almond or olive oil.
Andre Charles Boulle, cabinet on stands, 1680
Nowadays, one may frequently see fake tortoiseshell, made from plastic, such as celluloid or cellulos, which has the same density as the tortoiseshell, and which can be coloured with red, yellow and brown spots as to imitate real tortoiseshell. Technically, the distinction between real tortoiseshell and fake tortoiseshell is made via microscopic study, luminescence with long and short ultraviolet lights, by the study of the refraction index, or by a destructive test.